Methanol generation device, method for generating methanol, and electrode for generating methanol

ABSTRACT

The present invention provides a methanol generation device for generating methanol by reducing carbon dioxide, comprising: a container for storing an electrolyte solution containing carbon dioxide; a cathode electrode disposed in the container so as to be in contact with the electrolyte solution; an anode electrode disposed in the container so as to be in contact with the electrolyte solution; and an external power supply for applying a voltage so that a potential of the cathode electrode is negative with respect to a potential of the anode electrode. The cathode electrode includes a region of Cu 1-x Au x  (0&lt;x&lt;1). The anode electrode includes a region of a metal or a metal compound.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No.PCT/JP2014/003030, with an international filing date of Jun. 6, 2014,which claims priority of Japanese Patent Application No. 2013-135824,filed on Jun. 28, 2013, the contents of which are hereby incorporated byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to a methanol generation device, a methodfor generating methanol, and an electrode for generating methanol.

2. Description of the Related Art

Japanese Patent Application laid-open Publication No. 2000-254508A,Japanese Patent Application laid-open Publication No. Hei 1-313313A,U.S. Pat. No. 5,234,768, Japanese Patent Application laid-openPublication No. 2004-176129A, and Y. Hori, “Modern Aspects ofElectrochemistry”, 2008, vol. 42, p.p. 89-189 disclose a method forreducing carbon dioxide.

Japanese Patent Application laid-open Publication No. 2000-254508A andJapanese Patent Application laid-open Publication No. Hei 1-313313Adisclose a method for reducing carbon dioxide using a gas phase reactionperformed under high temperature.

U.S. Pat. No. 5,234,768 discloses a method for reducing carbon dioxideelectrochemically using a phthalocyanine metal complex.

Japanese Patent Application laid-open Publication No. 2004-176129A, andY. Hori, “Electrochemical CO₂ Reduction on Metal Electrodes”, ModernAspects of Electrochemistry, 2008, vol. 42, p.p. 89-189 disclose amethod for reducing carbon dioxide electrochemically using metal copperor copper halide.

SUMMARY

The present invention provides a methanol generation device forgenerating methanol by reducing carbon dioxide, comprising:

a container for storing an electrolyte solution containing carbondioxide; a cathode electrode disposed in the container so as to be incontact with the electrolyte solution;

an anode electrode disposed in the container so as to be in contact withthe electrolyte solution; and

an external power supply for applying a voltage so that a potential ofthe cathode electrode is negative with respect to a potential of theanode electrode,

wherein

the cathode electrode includes a region of Cu_(1-x)Au_(x) (0<x<1); and

the anode electrode includes a region of a metal or a metal compound.

The present invention provides a methanol generation device having highmethanol generation efficiency.

Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic view of a methanol generation device accordingto the present disclosure.

FIG. 1B shows a schematic view of a methanol generation device accordingto the present disclosure.

FIG. 1C shows a schematic view of a methanol generation device accordingto the present disclosure.

FIG. 2 is a graph showing results of the inventive example 1 and thecomparative examples 1-2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the methanol generation device and method forgenerating methanol according to the present disclosure will bedescribed with reference to the drawings.

Methanol Generation Device

FIG. 1A-FIG. 1C each show a schematic view of a methanol generationdevice for generating methanol by reducing carbon dioxide.

A methanol generation device 10 comprises a container 11, a cathodeelectrode 12, an anode electrode 13, and an external power supply 14. Asshown in FIG. 1A, the methanol generation device 10 may have a voltagemeasurement device 15 and a current measurement device 16, each of whichis used to monitor how carbon dioxide is reduced.

An electrolyte solution 17 is stored in the container 11. As oneexample, the electrolyte solution 17 is a potassium chloride aqueoussolution, a sodium chloride aqueous solution, a sodium hydrogencarbonate aqueous solution, or a sodium sulfate aqueous solution. Inparticular, a potassium chloride aqueous solution or a sodium chlorideaqueous solution is desirable. It is desirable that the electrolytesolution 17 has a concentration of not less than 0.05 mol/L and not morethan 5.0 mol/L.

The cathode electrode 12 has CuAu which is a copper-gold alloy.Hereinafter, copper is referred to as “Cu” and gold is referred to as“Au”. CuAu has a composition formula of Cu_(1-x)Au_(x) (where 0<x<1).

It is desirable that CuAu is in a state of a solid solution or anintermetallic compound. The solid solution means an alloy in which theelements constituting the alloy are randomly mixed at the atomic level.The intermetallic compound means a compound in which the elementsconstituting the compound are arranged regularly at the atomic level.

An example of a method for fabricating CuAu in the state of the solidsolution is a vacuum melting method or an arc melting method. The valueof x in the composition formula Cu_(1-x)Au_(x) represents thecomposition ratio of Au to Cu, both of which constitute CuAu. It isdesirable that the value of x is not less than 0.001 and not more than0.10. In particular, it is more desirable that the value of x is notless than 0.005 and not more than 0.05.

CuAu which constitutes the cathode electrode 12 may contain elementsother than Cu and Au, as long as the crystalline structure of CuAu isfree from disturbance. CuAu fabricated by a vacuum melting method or anarc melting method may contain impurities at a normally acceptablelevel. The crystalline structure can be observed, for example, byconducting an X-ray diffraction measurement.

The cathode electrode 12 may be composed only of CuAu; however, thecathode electrode 12 may have a stacked structure having a CuAu layerand a substrate for supporting the CuAu layer. For example, the cathodeelectrode 12 is a stacked structure obtained by forming a CuAu thin filmon the substrate formed of glass or glassy carbon. Instead, the cathodeelectrode 12 may be formed by arranging a lot of CuAu fine particles ona conductive substrate. The cathode electrode 12 is not limited, as longas the cathode electrode 12 is capable of reducing carbon dioxide andgenerating methanol.

The cathode electrode 12 is in contact with the electrolyte solution 17.More accurately, CuAu contained in the cathode electrode 12 is incontact with the electrolyte solution 17. As long as CuAu is in contactwith the electrolyte solution 17, only a part of the cathode electrode12 has to be immersed in the electrolyte solution 17.

The anode electrode 13 has a conductive material. An example of theconductive material is carbon, platinum, gold, silver, copper, titanium,iridium oxide, or an alloy thereof. The conductive material is notlimited, as long as the conductive material is not decomposed throughthe oxidation reaction of the conductive material itself.

The oxidation reaction of water performed in the anode electrode 13 isindependent of the reduction reaction of carbon dioxide performed in thecathode electrode 12. The material of the anode electrode 13 does nothave an influence on the reaction performed on the cathode electrode 12.

The anode electrode 13 is also in contact with the electrolyte solution17. More accurately, the conductive material included in the anodeelectrode 13 is in contact with the electrolyte solution 17. As long asthe conductive material is in contact with the electrolyte solution 17,only a part of the anode electrode 13 has to be immersed in theelectrolyte solution 17.

As shown in FIG. 1A, the methanol generation device 10 may have a tube18 in the container 11. Carbon dioxide may be supplied through the tube18 to the electrolyte solution 17. One end of the tube 18 is immersed inthe electrolyte solution 17.

As shown in FIG. 1B, a methanol generation device 100 may comprise asolid electrolyte membrane 19 in the container 11. The solid electrolytemembrane 19 divides the electrolyte solution 17 into an anode-sideelectrolyte solution 17L and a cathode-side electrolyte solution 17R. Inother words, the solid electrolyte membrane 19 divides the inside of thecontainer 11 into an anode container for storing the anode-sideelectrolyte solution 17L and a cathode container for storing thecathode-side electrolyte solution 17R.

The solid electrolyte membrane 19 prevents the anode-side electrolytesolution 17L and the cathode-side electrolyte solution 17R from beingmixed with each other. Since the solid electrolyte membrane 19 allowspassage of protons therethrough, the solid electrolyte membrane 19connects the cathode-side electrolyte solution 17R with the anode-sideelectrolyte solution 17L electrically. An example of the solidelectrolyte membrane 19 is a Nafion membrane available from Du Pont. Thereason why the electrolyte solution 17 is divided using the solidelectrolyte membrane 19 will be described later.

As shown in FIG. 1C, a methanol generation device 200 may comprise areference electrode 20 near the cathode electrode 12. The referenceelectrode 20 is in contact with the cathode-side electrolyte solution17R. The reference electrode 20 is used for measuring the electricpotential of the cathode electrode 12 and is connected to the cathodeelectrode 12 through the voltage measurement device 15. An example ofthe reference electrode 20 is a silver/silver chloride electrode(hereinafter, referred to as “Ag/AgCl electrode”).

Method for Generating Methanol

A method for generating methanol using the above-mentioned methanolgeneration device will be described below.

The methanol generation device may be placed under room temperature andatmospheric pressure.

A voltage is applied to the cathode electrode 12 using the externalpower supply 14 so that the voltage is negative with respect to thepotential of the anode electrode 13. A voltage exceeding a threshold forestablishing the methanol generation reaction has to be applied usingthe external power supply 14. The threshold varies depending on theinterval between the cathode electrode 12 and the anode electrode 13,the materials constituting the cathode electrode 12 and the anodeelectrode 13, and the concentration of the electrolyte solution 17. Itis desirable that the threshold is not less than 2.5 volts.

A part of the electric energy applied to the cathode electrode 12 withrespect to the anode electrode 13 is consumed for the oxidizationreaction of water which occurs on the anode electrode 13. For thisreason, in the case of using the methanol generation device shown inFIG. 1A or FIG. 1B, it is difficult to know the actual voltage appliedto the cathode electrode 12. On the other hand, in the case of using themethanol generation device shown in FIG. 1C, the actual voltage appliedto the cathode electrode 12 can be determined accurately. The potentialof the cathode electrode 12 with respect to the potential of thereference electrode 20 may vary depending on the materials of thereference electrode 20. It is desirable that the potential of thecathode electrode 12 with respect to the potential of the referenceelectrode 20 is not more than −1.7 volts.

As just described, by applying an appropriate voltage to the cathodeelectrode 12, the carbon dioxide contained in the electrolyte solution17 is reduced on the cathode electrode 12. As a result, methanol isgenerated on the surface of the cathode electrode 12.

On the other hand, oxygen is generated on the anode electrode 13 throughthe oxidization reaction of water. In a case where the electrolytesolution 17 contains methanol, not only water but also methanol isoxidized. In other words, a part of the methanol generated on thecathode electrode 12 may reach the anode electrode 13, and oxidized onthe anode electrode 13. As a result, the generated methanol is returnedto carbon dioxide. In order to prevent such a reverse reaction, as shownin FIG. 1B and FIG. 1C, the cathode-side electrolyte solution 17R andthe anode-side electrolyte solution 17L are preferably separated fromeach other using the solid electrolyte membrane 19.

A reaction current flows through the cathode electrode 12, since carbondioxide is reduced on the cathode electrode 12 and water is oxidized onthe anode electrode 13 by using the methanol generation device. As shownin FIG. 1A-FIG. 1C, the amount of the reaction current can be monitoredusing the current measurement device 16.

EXAMPLES

Hereinafter, the present disclosure will be described with reference tothe following examples.

Inventive Example 1 Fabrication of the Cathode Electrode

A cathode electrode formed of CuAu according to the inventive example 1was fabricated below.

First, Au was dissolved in Cu by a vacuum melting method so that x wasequal to 0.00167. Subsequently, the dissolved material was solidified.In this way, CuAu was obtained. The obtained CuAu was shaped into aplate having a length of 20 millimeters, a width of 20 millimeters, anda thickness of 2 millimeters. The surface of the plate was washed usingan organic solvent.

The composition of the CuAu plate was confirmed using an X-raydiffractometer. As a result, no peak of Au in the state of theelementary substance was observed. The formation of CuAu in which Au wasdissolved in Cu was observed.

And then, the obtained CuAu plate was adhered to a glass substrate. Inthis way, the cathode electrode according to the inventive example 1 wasfabricated.

Assembling of the Device

The methanol generation device shown in FIG. 1C was fabricated using theabove-mentioned cathode electrode. The elements of the methanolgeneration device are described below.

Cathode electrode: CuAu (Composition formula: Cu_(0.9833)Au_(0.0167))

Anode electrode: Platinum

Electrode interval: approximately 8 centimeters

Reference electrode: Ag/AgCl

Anode-side electrolyte solution: potassium hydrogen carbonate aqueoussolution having a concentration of 0.5 mol/L

Cathode-side electrolyte solution: potassium chloride aqueous solutionhaving a concentration of 0.5 mol/L

Solid electrolyte membrane: Nafion membrane (available from Du Pont,Trade name: Nafion 117)

Carbon dioxide was supplied through a tube to the cathode-sideelectrolyte solution by bubbling the cathode-side electrolyte solutionusing a carbon dioxide gas for thirty minutes (carbon dioxide flow rate:200 milliliters/minute).

Reduction of Carbon Dioxide

After carbon dioxide was dissolved in the cathode-side electrolytesolution, the container was sealed. Subsequently, a voltage was appliedusing a potentiostat so that the potential of the cathode electrode wasnegative with respect to the potential of the anode electrode. The valueof the applied voltage was controlled using the potentiostat so that thepotential of the cathode electrode with respect to the potential of thereference electrode was −1.9 volts.

After the voltage was applied for a certain period, the material andamount of the reaction products generated in the container weredetermined using gas chromatography and liquid chromatography. As aresult, carbon monoxide (CO), formic acid (HCOOH), methane (CH₄),ethylene (C₂H₄), aldehydes, and ethanol were detected as reductionproducts of carbon dioxide. Furthermore, as shown in FIG. 2, a peak wasobserved at the detecting position indicating the generation of methanol(i.e. the position indicated by the arrow depicted in the drawing) inthe product analysis of the head-space gas chromatography. In otherwords, it was confirmed that methanol was generated using the cathodeelectrode formed of CuAu.

In the inventive example 1, the production amount of methanol per 1000seconds of the electrolysis period was 3.1×10⁻⁷ mol/cm². Note that theelectrolysis period is equal to the period for which the voltage wasapplied to the cathode electrode using the external power supply.

The faraday efficiency of the methanol generation in the inventiveexample 1 was calculated. As a result, the calculated faraday efficiencywas 1.42%. Note that the faraday efficiency means a ratio of the chargeamount used for the generation of the reaction product to the chargeamount used for all the reactions. The faraday efficiency is calculatedin accordance with the following formula:

(Faraday efficiency of the methanol generation)=(the charge amount usedfor the generation of methanol)/(the charge amount used for all thereactions)×100 [%].

Comparative Example 1

An experiment similar to that of the inventive example 1 was performed,except that a Cu electrode containing no Au was used as the cathodeelectrode.

As a result, CO, HCOOH, CH₄, C₂H₄, aldehydes, and ethanol were detectedsimilarly to the inventive example 1. However, as shown in FIG. 2, nosignal was found at the detecting position of the methanol generation.In other words, methanol was not generated in the comparative example 1.

Comparative Example 2

An experiment similar to that of the inventive example 1 was performed,except that an Au electrode containing no Cu was used as the cathodeelectrode.

As a result, CO and HCOOH were detected. However, as shown in FIG. 2, nosignal was found at the detecting position of the methanol generation.In other words, methanol was not generated in the comparative example 2.

As just described, only in the case where the CuAu electrode was used,the generation of methanol was confirmed, as shown in FIG. 2. In otherwords, the present inventors believe that a special effect which causesmethanol to be generated occurs only in the case where the CuAuelectrode formed of an alloy of Cu and Au is used, and that such aspecial effect does not occur in the case where the Cu electrode or theAu electrode is used.

Inventive Example 2

An experiment similar to that of the inventive example 1 was performed,except that a sodium chloride aqueous solution having a concentration of0.5 mol/L was used as the cathode-side electrolyte solution.

As a result, it was confirmed that methanol was generated as thereduction product of carbon dioxide.

Inventive Example 3

An experiment similar to that of the inventive example 1 was performed,except that an electrode was used in which CuAu fine particles having asimilar composition ratio to CuAu of the inventive example 1 weresupported on a surface of a glassy carbon substrate.

As a result, it was confirmed that the obtained reaction products werealmost the same as those of the inventive example 1 and that methanolwas generated. Similar results were obtained in the case of using anelectrode in which a CuAu thin film having a similar composition ratioto that of the inventive example 1 was laminated on the glassy carbon.

Inventive Example 4

An experiment similar to that of the inventive example 1 was performed,except that a CuAu electrode formed of Cu_(0.995)Au_(0.005) was used asthe cathode electrode.

As a result, it was confirmed that methanol was generated as thereduction product of carbon dioxide.

Inventive Example 5

An experiment similar to that of the inventive example 1 was performed,except that a CuAu electrode formed of Cu_(0.95)Au_(0.05) was used asthe cathode electrode.

As a result, it was confirmed that methanol was generated as thereduction product of carbon dioxide.

Inventive Example 6

An experiment similar to that of the inventive example 1 was performed,except that the potentiostat was controlled so that the potential of thecathode electrode with respect to the potential of the referenceelectrode was −1.7 volts.

As a result, it was confirmed that methanol was generated as thereduction product of carbon dioxide.

Inventive Example 7

An experiment similar to that of the inventive example 1 was performed,except that the potentiostat was controlled so that the potential of thecathode electrode with respect to the potential of the referenceelectrode was −2.1 volts.

As a result, it was confirmed that methanol was generated as thereduction product of carbon dioxide.

Table 1 shows the comparison of the methanol generation amounts obtainedin the inventive examples 1-7 and the comparative examples 1-2. In thegraph shown in Table 1, the generation amount of methanol generated inthe inventive example 1 is set to be 100%. Each of the generationamounts of methanol generated in the inventive examples 2-7 and thecomparative examples 1-2 is indicated relatively.

TABLE 1 Methanol generation amount (relative amount, per unit chargeamount) Inventive example 1 100 Inventive example 2 98 Inventive example3 87 Inventive example 4 41 Inventive example 5 75 Inventive example 673 Inventive example 7 94 Comparative example 1 0 Comparative example 20

As shown in Table 1, it was confirmed that methanol was generated in theinventive examples 1-7 and that methanol was not generated in thecomparative examples 1-2.

As just described, it was confirmed that methanol was generatedefficiently as the reduction product of carbon dioxide on the cathodeelectrode by using the cathode electrode having a region of CuAu.

While the present invention has been described with respect to preferredembodiments thereof, it will be apparent to those skilled in the artthat the disclosed invention may be modified in numerous ways and mayassume many embodiments other than those specifically described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

INDUSTRIAL APPLICABILITY

The present disclosure provides a novel device and a novel method forgenerating methanol as the reduction product of carbon dioxide by usingthe cathode electrode having CuAu.

REFERENCE SIGNS LIST

-   10, 100, 200 methanol generation device-   11 container-   12 cathode electrode-   13 anode electrode-   14 external power supply-   15 voltage measurement device-   16 current measurement device-   17, 17L, 17R electrolyte solution-   18 tube-   19 solid electrolyte membrane-   20 reference electrode

1. A methanol generation device for generating methanol by reducingcarbon dioxide, comprising: a container for storing an electrolytesolution containing carbon dioxide; a cathode electrode disposed in thecontainer so as to be in contact with the electrolyte solution; an anodeelectrode disposed in the container so as to be in contact with theelectrolyte solution; and an external power supply for applying avoltage so that a potential of the cathode electrode is negative withrespect to a potential of the anode electrode, wherein the cathodeelectrode includes a region of Cu_(1-x)Au_(x) (0<x<1); and the anodeelectrode includes a region of a metal or a metal compound.
 2. Themethanol generation device according to claim 1, wherein theCu_(1-x)Au_(x) is a solid solution or intermetallic compound of Cu andAu.
 3. The methanol generation device according to claim 1, wherein thevalue of x is not less than 0.005 and not more than 0.05.
 4. Themethanol generation device according to claim 1, wherein the anodeelectrode is formed of carbon, platinum, gold, silver, copper, titanium,iridium oxide, or an alloy thereof.
 5. The methanol generation deviceaccording to claim 1, wherein the electrolyte solution is a potassiumchloride aqueous solution or a sodium chloride aqueous solution.
 6. Themethanol generation device according to claim 1, wherein an absolutevalue of the voltage is not less than 2.5 volts.
 7. The methanolgeneration device according to claim 1, further comprising: a referenceelectrode disposed in the container so as to be in contact with theelectrolyte solution, wherein the reference electrode has a region ofAg/AgCl; and the voltage to be applied by the external power supply tothe cathode electrode with respect to a potential of the referenceelectrode is not more than −1.7 volt.
 8. The methanol generation deviceaccording to claim 1, further comprising: a solid electrolyte membranefor dividing the container into a cathode container for storing a firstelectrolyte solution containing carbon dioxide and an anode containerfor storing a second electrolyte solution.
 9. The methanol generationdevice according to claim 8, wherein the first electrolyte solution is apotassium chloride aqueous solution or a sodium chloride aqueoussolution, and the second electrolyte solution is a potassium hydrogencarbonate aqueous solution, a sodium hydrogen carbonate aqueoussolution, or a potassium sulfate aqueous solution.
 10. A method forgenerating methanol using a methanol generation device, the methodcomprising: (a) preparing the methanol generation device comprising: acontainer; a cathode electrode; and an anode electrode, wherein thecathode electrode has a region of Cu_(1-x)Au_(x) (0<x<1); the anodeelectrode has a region of a metal or a metal compound; an electrolytesolution is stored in the container; the cathode electrode is in contactwith the electrolyte solution; the anode electrode is in contact withthe electrolyte solution; and the electrolyte solution contains carbondioxide; and (b) reducing the carbon dioxide contained in theelectrolyte solution by applying a voltage so that a potential of thecathode electrode is negative with respect to a potential of the anodeelectrode to generate methanol on the cathode electrode.
 11. The methodaccording to claim 10, wherein the methanol generation device is placedunder room temperature and atmospheric pressure in step (b).
 12. Anelectrode for generating methanol used for a methanol generation devicefor generating methanol by reducing carbon dioxide, comprising: a regionof Cu_(1-x)Au_(x) (0<x<1).